PHP Manual:
array_reduce

array_reduce

(PHP 4 >= 4.0.5, PHP 5)

array_reduce -- Iteratively reduce the array to a single value using a callback function

Description

mixed array_reduce ( array input, callback function [, int initial] )

array_reduce() applies iteratively the
function function to the elements of the
array input, so as to reduce the array to
a single value. If the optional initial is
available, it will be used at the beginning of the process, or as
a final result in case the array is empty.
If the array is empty and initial is not passed,
array_reduce() returns NULL.